1. Field of the Invention
The present invention relates to an optical scanning apparatus and optical scanning method and, more particularly, to an optical scanning apparatus for forming an electrostatic latent image on an image carrier by controlling the output from a light source and an optical scanning method applied to the optical scanning apparatus.
2. Description of the Related Art
A conventional electrophotographic image forming apparatus for executing image exposure using a laser beam irradiates a rotary polyhedral mirror (polygonal mirror) with a laser beam and scans and exposes a photosensitive member using the reflected light. At this time, the laser light source is preferably equidistant from the exposure surface of the photosensitive member independently of the rotational position of the polygonal mirror. That is, the exposure surface of the photosensitive member preferably forms a circular arc about the polygonal mirror. In fact, many image forming apparatuses employ a cylindrical photosensitive member from the viewpoint of image formation after exposure, and the linear portion of the cylinder corresponds to the main scanning direction of the laser beam. To solve the problem of nonuniformity of the optical path length from the laser light source to the surface of the photosensitive member caused by the cylindrical photosensitive member, conventionally, an optical unit called an f-θ lens with a complex structure is used to unify the exposure speed on the photosensitive member.
Along with the recent increase in image forming speed, an image forming apparatus which executes exposure using a plurality of laser light sources arranged in the sub-scanning direction is used. Even in this image forming apparatus using the plurality of laser light sources, it is necessary to equalize the main-scanning optical path lengths from the light source to the photosensitive member surface and also equalize the optical path lengths (scan lengths) between the photosensitive member and the plurality of lasers arranged in the sub-scanning direction. To meet these requirements, conventionally, the accuracies of the optical and mechanical structures are raised.
For example, the f-θ lens for an image forming apparatus is manufactured at a high accuracy. This inevitably increases the cost of the f-θ lens and makes it difficult to cope with cost reduction that is recently required of an image forming apparatus.
Additionally, as the resolution of the image forming apparatus increases, the conventionally allowed difference in scan length between the photosensitive member and the plurality of lasers influences the output image. That is, the scan length difference has become nonnegligible.
Furthermore, in an image forming apparatus having a plurality of photosensitive members, the structure for adjusting the scanning magnification (scan length) on each photosensitive member becomes complex or requires adjustment, resulting in an increase in cost.
To solve these problems, conventionally, an image forming apparatus as disclosed in, for example, Japanese Patent Laid-Open No. 2005-96351 has been proposed. In this image forming apparatus, an effective image area is divided into a plurality of areas along the longitudinal direction of the photosensitive member, and image data (piece of pixel) is inserted into or removed from each of the divided areas, thereby controlling the main-scanning partial magnification in each of the divided areas. This maintains the main-scanning magnification on the photosensitive member constant and prevents degradation in image quality.
However, the conventional image forming apparatus disclosed in Japanese Patent Laid-Open No. 2005-96351 has the following problems.
The problems of the conventional image forming apparatus disclosed in Japanese Patent Laid-Open No. 2005-96351 will be described here with reference to FIGS. 10, 11A, and 11B.
FIG. 10 is a block diagram showing the arrangement of a portion which executes image data (piece of pixel) insertion and removal in the conventional image forming apparatus disclosed in Japanese Patent Laid-Open No. 2005-96351.
In this image forming apparatus, a converting circuit 10 converts a density signal (density data) output from a memory 9 into a PWM turn-on pattern (laser turn-on pattern) and inputs it to a shift register 11. The shift register 11 has a storing capacity capable of storing turn-on patterns of at least two pixels (the shift register 11 shown in FIGS. 11A and 11B can store turn-on patterns of three pixels, as will be described later). The shift register 11 shifts image data of one pixel in synchronism with a clock signal output from a clock generation circuit (not shown). The shift register 11 outputs, to a laser drive circuit 12, the earliest one of the stored image data as a PWM turn-on pattern (laser turn-on pattern). The shift register 11 inserts or removes image data (piece of pixel) by using a predetermined shift method. This will be described with reference to FIGS. 11A and 11B. PWM is short for “pulse width modulation”.
FIGS. 11A and 11B are views showing the storage contents of the shift register 11. FIG. 11A shows the storage contents of the shift register 11 without insertion of image data (piece of pixel). FIG. 11B shows the storage contents of the shift register 11 with insertion of 1-bit image data (piece of pixel).
In this case, for example, one pixel is expressed by 4-bit image data. The shift register 11 has a storing capacity of 12 bits and can therefore store image data of three pixels. Each block shown in FIGS. 11A and 11B indicates a 1-bit storage area in the shift register 11. A number added to the right side of each block indicates an internal address.
The converting circuit 10 generates four image data bits of each pixel based on the density signal (density data) output from the memory 9 and supplies the image data to the shift register 11. Without insertion of image data (piece of pixel) (FIG. 11A), four image data bits D0(3) to D0(0) of the first pixel are inserted into areas of addresses 1 to 4, respectively. The image data bit D0(3) indicates the most significant bit of the first pixel. The image data bit D0(0) indicates the least significant bit of the first pixel. Next, four image data bits D1(3) to D1(0) of the second pixel are inserted into areas of addresses 5 to 8, respectively.
On the other hand, the four image data bits D0(3) to D0(0) of the first pixel are output to the laser drive circuit 12 in synchronism with the clock signal. The four image data bits D1(3) to D1(0) of the second pixel are shifted to the areas of address 1 to 4.
To insert image data (piece of pixel) next to the image data bits D1(3) to D1(0) of the second pixel, as shown in FIG. 11B, the image data bit D1(0) identical to the image data bit D1(0) stored in the area of address 4 is stored in the area of address 5. Then, four image data bits D2(3) to D2(0) of the third pixel are inserted into areas of addresses 6 to 9, respectively.
After that, the four image data bits D1(3) to D1(0) of the second pixel are output to the laser drive circuit 12 in synchronism with the clock signal. The image stored in the areas of addresses 5 to 9 are shifted to the areas of address 1 to 5.
In image data (piece of pixel) removal, 1-bit image data is removed (deleted) from the shift register 11.
In this way, the main-scanning partial magnification on the photosensitive member is maintained constant, and the degradation in image quality is prevented.
However, when such insertion of 1-bit image data (piece of pixel) is repeated, the capacity of the shift register 11 may become too small to store new image data received from the converting circuit 10. If the shift register 11 does not have a sufficient capacity, the converting circuit 10 must stop density data read-out from the memory 9 until a predetermined capacity is ensured. Additionally, when removal of 1-bit image data (piece of pixel) is repeated, image data to be supplied from the shift register 11 to the laser drive circuit 12 may run out. In this case, the converting circuit 10 must read out not image data of one pixel but image data of two pixels from the memory 9 simultaneously and supply PWM turn-on patterns (laser turn-on patterns) of two pixels to the shift register 11.
To stop density data read-out from the memory 9 or simultaneously read out density data of two pixels, a monitoring part that monitors the storing state in the shift register 11 is necessary. A monitoring result (shortage in capacity or lack of stored image data) obtained by the monitoring part is fed back to a reading control unit (not shown) which controls data read-out from the memory 9, thereby stopping density data read-out or simultaneously reading out density data of two pixels.
If a delay time exists in the signal transmission path from the monitoring part to the reading control unit, the monitoring part needs to predict an amount of change of the monitoring result during the delay time and notify the reading control unit of it. Hence, the structure of the monitoring part becomes complex.
If the monitoring part is designed and manufactured, and then, a new module (e.g., control unit for another image processing) is added between the monitoring part and the reading control unit by changing the specifications, the delay time also changes, and the monitoring part requires redesign.